Synaptic transmission controls information flow in the brain, and synaptic dysfunction is likely the biological basis of several neurodevelopmental disorders including autism spectrum disorders (ASDs), Rett syndrome, and neuropsychiatric disorders such as schizophrenia. Human genetic studies revealed that an increasing number of mutations in genes encoding synaptic adhesion proteins, such as the neuroligins (NLGNs), are linked to ASDs. Despite numerous animal studies, given the lack of a readily accessible source of primary neurons from subjects with autism, how mutations in these genes cause pathology and synaptic dysfunction in humans remains enigmatic. Using heterologous cell systems, we recently discovered that one autism-linked missense mutation, arginine (R) to cysteine (C) at position 451 of the human NLGN3 (NLGN3 R451C), disrupts protein trafficking and causes endoplasmic reticulum (ER) stress with activation of the Unfolded Protein Response (UPR), presenting a possible novel mechanism of autism etiology. This hypothesis has never been tested before in neurons and, more importantly, whether it translates to humans is not known. Fortunately, recently developed novel techniques in stem cell biology have made this analysis possible. Furthermore, it is now possible to create human neurons carrying gene mutations on an isogenic background, eliminating the genetic background noise that confounded previous stem cell research using samples from multiple individuals. Therefore, I propose to examine if ER stress and UPR are mechanistically linked with the synaptic dysfunction in human neurons carrying the R451C gene mutation on an isogenic background. I will investigate the sub-cellular localization of specific proteins involved in UPR and ER stress in parallel with putatively correlating pathways. An immediate and functional readout of neuronal phenotypes will be provided by morphometric and electrophysiological analyses. Mechanistic studies using small molecule chaperones and inhibitors of the UPR will solidify the potential link between ER stress, UPR and synaptic dysfunction. The most significant impact of this study is that it has the potential to 1) uncover a novel pathogenic mechanism by which a key autism-linked mutation causes neuronal dysfunction and 2) determine the roles of ER stress and the UPR in regulating synaptic transmission in a human neuronal context.